Typically, watermarks are read in white (ambient) light using a color imager that can discriminate the colors (Blue, Yellow, Red, Green, etc.). In addition, watermarks are read using a monochrome imager and monochrome illumination (typically red).
Watermarks encoded in the chroma space uses the chrominance component of an image's color space. In contrast, a classic watermark uses the luminance of an image. The classic watermark is detected based on the image's contrast. Typically, classic watermarks show up in black and white and the human eye is more sensitive to perceiving such watermarks. Chroma watermarks are more difficult to detect by the human eye due to the lack of color perceptibility.